Ratio of air and fuel, or air/fuel ratio, of the air/fuel mixture supplied to an internal combustion engine is controlled in accordance with the operating conditions of the vehicle to suppress noxious substances in the exhaust gas and/or to enhance the thermal efficiency of the engine. This control is performed using an oxygen sensor (O.sub.2 sensor) for detecting oxygen concentration of the exhaust gas and an air/fuel ratio control device for controlling the air/fuel ratio fo the air/fuel mixture. The oxygen sensor is employed to feedback control the air/fuel control device in dependence on the output of the O.sub.2 sensor, so that the air/fuel ratio of the air/fuel mixture is near the stoichiometric mixture ratio. The O.sub.2 sensor used in air/fuel ratio control is tested and evaluated for its output and response characteristics using a testing device.
Conventional testing devices include those which use combustion gas of a propane burner to test O.sub.2 sensors, and those which use exhaust gas of an actual engine to test O.sub.2 sensors. In the device using a propane burner, output and response characteristics of O.sub.2 sensors are evaluated using the combustion gas. The device using an actual engine uses a computer to control the composition of air/fuel mixture to higher fuel concentrations than the stoichiometric mixture ratio (rich condition) or lower fuel concentrations than the stoichiometric mixture ratio (lean condition), and output and response characteristics of O.sub.2 sensors are evaluated using exhaust gas close to that of actual vehicles under rich, lean, and transient conditions.
The O.sub.2 sensor using a propane burner, since it is not able to evaluate exact characteristics because the composition of the combustion gas differs from that of actual exhaust gas, is used only for confirmation of quality control of O.sub.2 sensors.
Evaluation of O.sub.2 sensors using exhaust gas of an actual engine is expected to provide the same results as evaluation of exhaust gas of actual vehicles. However, the engine varies in characteristics as it is operated for an extended period of time, and, if the engine is replaced with another one, evaluation results may differ even for the same O.sub.2 sensor, with poor test reliability. Furthermore, when the air/fuel ratio of mixture varies from rich to lean condition, or lean to rich condition, or when such variation occurs abruptly, conditions which are most important to evaluate the characteristics of O.sub.2 sensors, the engine must be operated at a high speed and under a heavy load. This adversely affects the engine and impairs the reproducibility of the test. Therefore, evaluation of O.sub.2 sensors has been reliable only when the engine is operated at a low speed and with a light load. Evaluation of characteristics of O.sub.2 sensors has been inaccurate when the air/fuel ratio of mixture varies largely or abruptly.
With a view to obviating the above prior art defects, it is a primary object of the present invention to provide a testing device for oxygen sensors which is able to create an exhaust gas of composition close to that of actual exhaust gas and containing constituents adversely affecting evaluation of O.sub.2 sensors, and, to simulate exhaust gas of various running conditions of the vehicle with good repeatability. This enables evaluation of O.sub.2 sensors under conditions close to those of actual driving conditions, and improves reliability of the test.